These sensors are used notably for the industrial inspection of products placed on a conveyor belt. Other applications are possible in the field of space observation for example.
One of the problems to which it is desired to afford a solution is the obtaining of an image signal of sufficient amplitude even when the relative movement of the object with respect to the sensor is very fast. It is indeed understood that if the movement is fast, the light integration time must be very brief, failing which the image would have only very poor resolution in the direction of the movement. But if the integration time is very brief, few photons are gathered by the pixels of the sensor. Active-pixel MOS technology is preferred because notably of the compatibility that it allows between the manufacture of the pixels proper and the manufacture of the peripheral electronic circuits which serve to control the sensor and to process the electronic image gathered.
Moreover, to improve the signal/noise ratio of the readout of an image, it has heretofore long been proposed to use multilinear strips operating by movement and signal integration (known as TDI sensors, the acronym standing for “Time Delay Integration”) in which an image of a line of points of an observed scene is enhanced through the addition of successive images taken by N photosensitive lines observing one and the same line of the scene successively as the scene moves past the sensor.
At constant exposure time, the sensitivity is improved in the ratio of the number N of lines. The signal-to-noise ratio is improved in a ratio that can range from the square root of N to N.
The production of a TDI sensor by CCD technology is relatively easy: the addition of the signals point-by-point is done simply by emptying into a line of pixels the charge generated and accumulated in the previous line, in synchronism with the relative displacement of the scene and of the sensor. The last line of pixels, having accumulated N times the charge engendered by the observed image line, may be read out.
The production of a TDI sensor by MOS technology is more difficult; it involves the production of pixels with multiple gates operating by transferring charge in the manner of CCD sensors but with a different technology exhibiting risks of poor transfers from one pixel to the next. These pixels are not active pixels, they comprise only charge storage and transfer means but no means for charge-voltage or charge-current conversion inside the pixel.
It may be pointed out moreover that the use of sensors with multiple lines of pixels may pose problems in the case of poor synchronization or of poor regularity of the movement of the observed scene; indeed, if the movement is not well synchronized or not very regular, the addition of signals seen by the various lines will be falsified.
According to the invention a solution is proposed for producing a very fast linear sensor, having improved sensitivity and an improved signal-to-noise ratio relative to an MOS technology sensor that only had one line of pixels, but without using complex technology for the production of the pixels.
The solution according to the invention relies on the use of only two lines of pixels, operating in TDI mode but nevertheless using active pixels with charge-voltage conversion within the pixel. According to the invention, the pixels of same rank of the two lines each use a photodiode and a node for charge storage with a transfer gate adjacent to the photodiode and to the storage node so as to transfer the charge accumulated in the photodiode to the charge storage node. The storage node is shared between the two pixels of same rank, and the charge of the two photodiodes is transferred successively into this node before the reading of the potential taken by the node on account of this charge; the time interval which separates the two charge transfers corresponds substantially to the time which separates the transit of one and the same image line successively past the two lines of pixels.
Thus, the storage node internal to the pixel but common to two pixels sees the successive arrival of two packets of electrical charge, arising from two different photodiodes but which see one and the same image point in the relative movement between the observed scene and the sensor.
Consequently, it has been found that when limiting oneself to two lines of pixels, therefore to a modest gain in terms of sensitivity and signal-to-noise ratio, there is no necessity to appreciably modify the technology for producing the pixels: the latter can remain conventional MOS technology active pixels (typically pixels with one photodiode and a few transistors).